Engineering Geology Projects
MACKAYE HARBOR ROAD RELOCATION PROJECT
Plateau Geoscience Group performed a geologic evaluation of alternative proposed road alignments for MacKaye Harbor Road in the face of continued climate change and expected rise in sea level. The MacKaye Harbor Road project is located at the southern end of Lopez Island, Washington. MacKaye Harbor Road is a the main paved road providing access to residences, Agate Beach County Park, and Iceberg Point National Monument, and is situated slightly above mean sea level with road instability resulting from high tide wave erosion. The project area is underlain by Mesozoic metasedimentary rocks, Peistocene continental glacial drift, and Quaternary beach and peat deposits. The project terrain includes uplands underlain by bedrock, low-lying areas identified in the National Wetland Inventory Map as wetlands that include peat deposits, and shoreline areas. Six road relocation alternative routes were evaluated that would provide area access with a projected 12-foot rise in sea level. Recommendations were provided for each of the relocation alternatives regarding unsuitable soil, potential slope instability, hydrologic conditions, and modifications to proposed road alignments.
One proposed alternate road alignment in transitional terrain north of bedrock terrain underlying Iceberg Point.
View north of west margin of Outer Bay potentially inundated with rising sea level.
Supratidal area west of Aleck Bay potentially inundated with rising sea level.
BELLS MOUNTAIN ROAD/CULVERT ASSESSMENT
Plateau Geoscience Group conducted an evaluation of two culverts on a tributary stream to Rock Creek, on the Bells Mountain Development property near Heisson, Washington. The two culverts have undergone significant deformation resulting from incipient failure of the road fill crossing the stream. Washington Department of Fish and Wildlife identified the two culverts as have 0% fish passability for this stream that contains protected fish species and must be repaired. Observations made during a site visit indicated deformation and breakage of the corrugated metal culverts, and soil erosion around and beneath the inlet and outlet ends of the culverts resulting in a foot or more elevation difference between the culvert inverts and the stream water surface. Recommendations were provided regarding relative urgency between the two culverts for proposed future repairs.
Road bed eroded around culvert. Culvert invert above water level.
Culvert deformed and displaced by road bed settlement.
KOSLOSKI PROPERTY CRITICAL AREA ASSESSMENT
Plateau Geoscience Group conducted a Geologic Hazard Assessment of the Kosloski property, located north of Pacific Way near Longview, Washington, on the south-facing slope of the Willapa Hills. The Geologic Hazard Assessment was in support of timber harvest and land development permits. The property is underlain by the Cowlitz Formation and Columbia River Basalt Formation, and is in the eastern portion of an identified large dormant deep-seated landslide. The property owner proposed building sites for construction of nine single-family residences , and timber harvesting associated with residential development. The assessment evaluated geologic conditions in the areas of proposed residential development and provided recommendations regarding development setbacks, design standards, and feasibility of the potential development with respect to identified landslides and landslide features.
Proposed single family residence building site.
Landslide terrain located within a large dormant deep-seated landslide.
HALBLIEB PROPERTY GEOHAZARD ASSESSMENT
Plateau conducted a geohazard assessment of the to observe post-construction conditions and to determine whether the new shop building complies with recommendations made in the previous 2007 geohazard assessment report as required by Cowlitz County. The site is located off Clark Creek Lane near Longview, Washington. The post-construction geohazard assessment results indicated that the new construction was in compliance with soil cuts, drainage, and erosion control recommendations.
DOBBE PROPERTY HAZARD ASSESSMENT
A geologic hazard assessment was made for this property as required by Cowlitz County for issuance of a building permit. The property is located in an area where dormant landslides have been identified.
LITTLE KALAMA ROAD LANDSLIDE ASSESSMENT
Plateau Geoscience Group conducted a geologic hazard assessment of a property adjacent to Little Kalama River Road, west of Longview, Washington. The assessment included review of available geologic information, a site visit, and calculation of factor of safety with respect to potential future slope movement to meet Cowlitz County land development requirements. No evidence of slope movement was observed on the property but potential slumping was obaserved along Little Kalama Road adjacent to the property.
View south along Little Kalama Road, adjacent to site. Ongoing road deformation visible at edge of road left of photo center.
FITZGERALD PROPERTY GEOHAZARD ASSESSMENT
A Geologic Hazard Assessment was conducted at the Fitzgerald property as required by Clatsop County Ordinance 80-14 for issuance of a building permit for construction of a single-family dome residence. County Ordinance requires evaluation of identified potential geologic hazards for the Cannon Beach, Oregon, area including mass wasting, wave attack, compressible soil, seismicity, and wind and soil erosion. The property is located in the hills east of Cannon Beach and is underlain by the Cannon Beach member of the Astoria Formation. Landslide terrain was observed on the northern portion of the property, but no recent or ongoing mass wasting was observed in the area proposed for construction. Two relevant potential geologic hazards identified for the property include landslide and earthquake ground shaking. A building permit was issued based on the results of the geologic hazard assessment. A follow-up assessment was recently conducted to comply with the County five-year requirement for updating assessments.
BRADFORD PROPERTY HAZARD ASSESSMENT
Plateau Geoscience Group performed a Geologic Hazard Assessment of the Bradford property, located in Longview, Washington, as required by Cowlitz County Department of Building for issuance of a building permit for construction of a new pole barn. The property is on the south-facing slopes of the Willapa hills in an area mapped as a large ancient landslide that extends some 6,000 feet east to west. The landslide is typical for the area with rotational movement in the headscarp area and translational movement toward the foot of the landslide mass. The hazard assessment concluded that construction of a new pole barn was feasible with setbacks from downhill slopes.
MEDEROS PROPERTY GEOHAZARD ASSESSMENT
Plateau conducted a geologic reconnaissance of the 20-acre property to identify any geohazards that may affect residential development. The property owner intended to subdivide into four 5-acre parcels with new homesites to be located on three of the parcels. Plateau evaluated surface water, slopes and potential unstable slopes on the three homesite parcels and provided recommendations for future development of the homesite parcels.